This invention relates to a voltage-controlled attenuator which is made low in noise and distortion.
A conventional voltage-controlled attenuator is arranged as shown in FIG. 1.
More specifically, an input voltage E.sub.IN is converted into a current by a differential amplifier having transistors Q.sub.1 and Q.sub.2, and the current is used to drive the common emitters of a pair of differential amplifiers which include transistors Q.sub.3 and Q.sub.4, and Q.sub.5 and Q.sub.6, respectively. The base biases of the transistors Q.sub.3 through Q.sub.6 forming the pair of differential amplifiers are controlled by a voltage Vc, so that the collector current of one of the transistors Q.sub.3 and Q.sub.4 is increased as the collector current of the other is decreased, and so that the collector current of one of the transistors Q.sub.5 and Q.sub.6 is increased while that of the other is decreased.
That is, the currents in the load resistors R.sub.1 and R.sub.2 are controlled, and the output voltages are also controlled by the bias voltage.
Let us consider the operation when the transistors Q.sub.4 and Q.sub.5 are conductive (on), i.e. where the attenuation factor of the attenuator is 0 dB. The distortion and noise characteristics are determined by the transistors Q.sub.1 and Q.sub.2, the load resistors R.sub.1 and R.sub.2 are emitter resistors R.sub.3 and R.sub.4 only. That is, the distortion characteristic is determined by the steady voltage V.sub.R applied to the emitter resistors R.sub.3 and R.sub.4 and the ratio of the V.sub.BE characteristics KT/q of the transistors Q.sub.1 and Q.sub.2. The noise characteristic is governed by the square by the square average value of the terminal noises Vn=.sqroot.4KTBR of the load resistors R.sub.1 and R.sub.2 and the emitter resistors R.sub.3 and R.sub.4.
Accordingly, a level which meets both distortion and noise characteristics cannot be obtained without increasing the current value of the constant current source I. However, increasing the current of the constant current source I is disadvantageous in that the transistors must be increased in size.